With the spread of cellular phones and optical communication, ceramic substrates have been employed as substrates for mounting semiconductor devices of high output and high power consumption which work at the high-frequency band, such as GaAs-based FET, Si—Ge-based HBT, Si-based MOSFET and GaN-based laser diode, because of low dielectric loss at high frequency. Of the ceramic substrates, a sintered aluminum nitride substrate has been particularly paid attention because it has a high thermal conductivity and has a thermal expansion coefficient close to that of a semiconductor device.
In order to bond a device onto the ceramic substrate such as a sintered aluminum nitride substrate, a process comprising forming a first undercoating metal layer that is firmly bonded to the ceramic substrate and a second undercoating metal layer by metallization, then forming an electrode layer composed of a noble metal such as Au, Ag, Pd or Pt on the undercoating metal layer and then soldering a device to the noble metal electrode layer is generally carried out. For the electrode layer, Au is preferably employed because it has extremely low resistance to conduction and has excellent wire bonding property. In order to solder the device, reflowing is frequently adopted from the viewpoint of efficiency, and on that account, a solder layer pattern for bonding the device needs to be formed in advance on the electrode layer of the substrate. In such a substrate for reflowing, highly precise formation of a solder layer in a fine region on the substrate using thin film technique has become necessary with promotion of high integration of semiconductor devices, and the solder layer is generally formed by laminating various metal thin film layers so as to give a prescribed solder composition when they are melted. In this specification, such a solder is referred to as a “thin film laminated structure solder” hereinafter, and a ceramic substrate having an electrode layer and a pattern of the thin film laminated structure solder formed on the electrode layer is also referred to as a “ceramic substrate with a solder layer” simply hereinafter.
As the ceramic substrate with a solder layer, a substrate using, for a solder layer, a thin film laminated structure solder of Au—Sn-based (see patent documents 1 and 2), or a substrate using, for a solder layer, a thin film laminated structure solder which gives a Sn-37 wt % Pb eutectic solder having a melting point of 183° C. or gives a solder consisting of this eutectic solder and a slight amount of a different metal or the like (both solders are generically referred to as an “Sn—Pb eutectic solder” hereinafter) (see patent document 3), is known. The Sn—Pb eutectic solder is a most general solder as an electronic industrial solder and is widely employed, and even the thin film laminated structure solder (e.g., solder consisting of Pb thin films and Sn thin films which are alternately laminated) can bond a device with high bond strength.
On the other hand, harmfulness of lead has become a problem, and a solder containing no lead component, namely, a so-called Pb-free solder, has been employed. As the Pb-free solder, a solder having a melting point equivalent to that of the Sn—Pb eutectic solder is desired from the viewpoint of substitution for the Sn—Pb eutectic solder, and as such a Pb-free solder, a tin-rich Au—Sn solder described in the aforesaid patent document 1 is known.
Patent document 1: Japanese Patent Laid-Open Publication No. 373960/2002
Patent document 2: Japanese Patent Laid-Open Publication No. 192581/1999
Patent document 3: Japanese Patent Laid-Open Publication No. 186884/1993
However, it has become apparent that in the case where a device is soldered to the ceramic substrate with a solder layer described in the patent document 1, specifically a ceramic substrate having an Au electrode layer, a layer of at least one metal selected from the group consisting of Ag, Cu and Ni (referred to as a “barrier metal layer” hereinafter) which is arranged on the Au electrode layer, and a solder layer containing Sn or In as a main component, particularly a solder layer composed of metals containing Sn or In as a main component and having an Au content of less than 20% by weight, which is arranged on the barrier metal layer, there resides a problem that the bond strength is lowered a little after a reliability test (specifically, heat cycle test wherein the resulting device-bonded substrate is exposed to a temperature of −55° C. and a temperature of 125° C. repeatedly) though the initial bond strength is excellent.
It is an object of the present invention to provide a ceramic substrate with a solder layer, which has a Pb-free thin film laminated structure solder and has highly reliable bond strength.